1. Field of the Invention
The present invention relates to video tape recorder heads, more particularly to a method and apparatus for rapidly switching a read head to a read mode once a write operation has been carried out.
2. Discussion of the Related Art
Various types of video tape recorders allow for counting a tape's length in real time. To achieve this function, while the tape is being recorded, a rectangular signal, having approximately a 25-Hz frequency, is simultaneously recorded by a control head on a control track of the tape. Thus, when the tape is played back, the rectangular control signal provides a real time count of the length of the tape, independent of the tape speed (fast search, slow motion, normal reading mode . . . ). Additionally, some video tape recorders use the control signal in order to simultaneously record marks on the tape, which are also referred to as indices. The beginning of a sequence marked by the index may then be searched for, for example, in a fast search mode.
FIG. 1 shows an exemplary rectangular control signal C recorded on a tape and a waveform H output by a read head reading the signal C.
The control signal C, as shown in FIG. 1, is cyclical. As is shown in FIG. 1, the first two periods of the signal have a 50%-duty cycle corresponding to a situation where the tape is not marked with an index. For the third period, signal C has a 27.5%-duty cycle, and for the fourth period a 60%-duty cycle. The 27.5% and 60% duty cycles are standardized ratios corresponding to binary values "1" and "0", respectively. Thus, in order to mark the tape with a digital index of n-bits, a combination of 27.5% and 60% duty cycle signals is recorded for n successive periods of signal C.
The control head behaves like a derivator, deriving the control signal C. Thus, as shown, signal H provided by the control head comprises a positive pulse at each rising edge of signal C and a negative pulse at each falling edge of signal C. The rising edges of signal C are separated by a constant length period. By detecting the positive pulses of signal H (when a positive threshold Vs is exceeded), the tape's playing time can be counted in real time. In addition, the falling edges of signal C are modulated with respect to the rising edge and correspond to the above-mentioned duty cycles. Thus, by detecting the negative pulses of signal H (when a negative threshold -Vs is exceeded), it is possible to check whether the control signal C includes binary data 0, 1, or not.
The marking of an index on the tape when, for example, a user desires to spot a sequence on the tape, is made as follows. A marking circuit, operating in a read mode, detects a rising edge of signal C, switches to a write mode, writes a high value during the required time interval (corresponding to 27.5% or 60% of the period), and then writes a low value, at least during a time interval corresponding to 60% of the period in case a high value was previously written during 27.5% of the period. Then, the marking circuit must rapidly switch again to the read mode in order to read the next rising edge of signal C. The transition from the write mode to the read mode raises problems for demagnetizing the read head and stabilizing a read amplifying circuit so that parasitic pulses are not recorded on the tape and so that the next rising edge of the control signal is correctly detected. These problems are described in more detail with reference to FIG. 2.
FIG. 2 is a schematic diagram of a conventional control circuit including the read head 10. A terminal A of the read head 10 is connected to a non-inverting input V+ of a high gain differential amplifier 12, and a second terminal B of the read head 10 is connected to the inverting input V- of amplifier 12. Input V+ of amplifier 12 is biased by a resistor R1 connected to a reference voltage Vref which is set to be approximately equal to half a supply voltage VCC. Input V- of amplifier 12 is further connected to ground through a capacitor C1 and to an output H of the amplifier 12 through a resistor R2. The specific connections of amplifier 12 regulate a quiescent voltage output by the amplifier to a value approximating Vref, independent of an offset voltage input to the amplifier. A time constant R2.times.C1 of the resistor, capacitor combination is selected so as to obtain, across capacitor C1, a constant voltage substantially equal to the mean output voltage H of amplifier 12.
Output H of amplifier 12 is connected to a hysteresis comparator 14 having a high threshold value Vs and a low threshold value -Vs, as shown in FIG. 1. The comparator 14 detects the positive and negative voltage pulses of signal H. If the quiescent voltage of amplifier 12 was not regulated as described above, it could vary as a function of the input offset voltage which, in turn, significantly varies as a function of temperature. Without such regulation, the comparator 14 might not be able to detect the positive or negative pulses of signal H. The output of the comparator 14 is used by counting circuits (not shown) for counting, in real time, the unwinding of the tape and for detecting the indices recorded on the tape.
Terminal B of the read head 10 is also connected to voltage Vref through a switch S1. Terminal A is also connected to voltage VCC through a switch S2 and to a current source I1 through a switch S3. Additionally, terminal A is connected to a switch S4 connected in series to an adjustable current source I2. The adjustable current source I2 is controlled by an inverting amplifier 16, an input of the inverting amplifier being connected to the voltage supply VCC through a current source I3 and to ground through a capacitor C2. Capacitor C2 can be shorted to ground by a switch S5.
Resistor R2 can be shorted through a switch S6.
The output of comparator 14 can be set to a high impedance through a switch S7.
Switches S1-S7 are controlled by a control circuit (CTRL) 18, in a way described hereinafter, to set the head to read or write mode and, in the write mode, to record on the tape a binary value "0" during the activation of a signal B0, or to record a binary value "1" during the activation of a signal B1. The sequencing of switches S1-S7 is determined by the control circuit 18 as a function of a voltage across capacitor C2, wherein the capacitor C2 together with source I3 and switch S5 comprises a voltage ramp generator.
FIG. 3 shows the waveforms of various signals that appear in the circuit of FIG. 2 when it is desired to write a binary "0" value on the tape. The given example corresponds to the writing of a square wave having a 60% duty cycle, that is, to the writing of a binary value "0".
Referring to FIG. 3, curve B0 represents the wave-form of signal B0 for the writing of a binary "0". Curve C represents the waveform of the control signal to be recorded on the tape. Curves S1-S7 represent the control signals to be applied to switches S1-S7, respectively, a high level of the signals corresponding to a switched-on condition. Curve VC2 represents the voltage across the ramp generator capacitor C2. Curve VH represents, in solid lines, the voltage signal V+ at the non-inverting input of amplifier 12 (terminal A), and, in dashed lines, the voltage signal V- at the inverting input of amplifier 12 (terminal B). The voltage V- also represents the voltage signal across the capacitor C1. Curve I.sub.10 represents the current in the read head 10. Curve H represents the output voltage signal of amplifier 12.
The time interval during which signal C is at a high level (corresponding to 27.5% or 60% of the period of signal C) is not to scale, as shown in FIG. 3, in order to better show the major events occurring for the last 40% of the period of signal C.
At a time t0, an order to write a "0" is provided to the tape in the form of a pulse B0. At t0, the control circuit is in the read mode (signals S1-S6 are at a low level and signal S7 at a high level) and remains in the read mode until the occurrence of the next rising edge of the control signal C, which occurs at a time t1.
At a time t2, prior to the possible occurrence of a falling edge of the control signal C (corresponding to 27.5% of the period of signal C after time t1 for a recorded binary value 1), the write mode of the tape starts operating. Simultaneously, switch S7 is turned off so that the variations of the output signal H of amplifier 12 are not taken into account during this write mode and during a subsequent transition mode to be discussed below. The write mode comprises two phases, labeled as 1 and 2 in curve I.sub.10.
Phase 1 includes allowing a positive current to flow through head 10 so as to write a high level signal on the tape. To achieve this function, switch S1 is turned on in order to connect terminal B of the read head 10 to the voltage Vref; switch S2 is turned on in order to connect terminal A of the read head 10 to the voltage supply VCC. Thus, a voltage VCC-Vref is applied across head 10. Switch S1 is turned on in order to maintain a constant voltage across capacitor C1 so that the voltage is not affected by any subsequent variations of the output voltage H of amplifier 12. Before switch S1 is biased on, voltage V- across capacitor C1 is, as shown by curve V.sub.H, slightly higher than voltage Vref. The voltage applied to the read head 10 during this phase generates a positive current in the read head 10 which writes on the tape a high level value of the control signal. Phase 1 lasts up to a time t3 occurring, in the present example, at a time corresponding to 60% of the period of signal C. In the alternative, for the write operation of a binary value 1 on the tape (not shown), time t3 occurs at a time corresponding to 27.5% of the period of signal C.
At a time t3, phase 2 of the write mode begins. Phase 2 consists in writing a falling edge of the control signal C. In all cases (writing of a binary value 0 or 1 on the tape), phase 2 may be slightly prolonged beyond 60% of the period of signal C, for example, up to 61%. Alternatively, during the writing operation of a binary value 1 (not shown), phase 2 lasts from 27.5% to 60% of the period of signal C in order to erase a possible binary value 0 that would have been previously recorded. Switch S2 is turned off and switch S3 is turned on. Switch S3 connects terminal A of the read head 10 to ground through the current source I1. Thus, a negative current is imposed on the read head 10 that writes a binary "0" value on the tape. Voltage V+ drops to a low value corresponding to the flow of a negative current through the head.
At a time t4, the write mode stops and a transition mode starts in order to rapidly switch from the write mode to the read mode before the occurrence of the next rising edge of signal C. The transition mode comprises three successive phases, labeled as 3 to 5 in curve I.sub.10. Phase 3 is an undesired latency phase. After the write mode, a current in the inductive component of head 10 must be restored sufficiently slowly to a zero value so that no overvoltage is generated and no parasitic pulse is written on the tape. To achieve this function, it is desirable to impose to the read head 10 a current ramp having a decreasing absolute value. At time t4, switch S5 is turned off, thereby allowing capacitor C2 to be charged by the current source I3. Voltage VC2 across capacitor C2 thus starts increasing linearly. At time t4, switch S3 is also turned off, and switch S4, which connects the variable current source I2 to terminal A of the head 10, is turned on. Since the variable current source I2 is negatively controlled by voltage VC2, the current I2 decreases in absolute value as a function of increasing voltage VC2. Current I2 decreases from a maximum theoretical value I2max selected as close as possible to value I1 of the current present in head 10 at the end of phase 2. However, the value I2max must be chosen higher than I1 in order to prevent, due to manufacturing differences and temperature variations, the current I2max from becoming lower than I1, which would generate a current step in the head 10 and result in a writing of a parasitic pulse on the tape. Thus, at time t4, and until current I2 reaches value I1, source I2 is saturated, that is, the head 10 consumes a current lower than the current that source I2 tends to provide. Since the slope of current 12 is low, phase 3 is long-lasting.
At a time t5, the current of source I2 reaches value I1, and phase 4 provided for effectively discharging the current accumulated in the head 10 starts.
At a time t6, the current in head 10 is cancelled. Time t6 is approximately detected when voltage VC2 reaches an adequately selected threshold value V1. At time t6, the voltage signal V+ is beyond its initial value and voltage V- (that is, the voltage across capacitor C1) is below its initial value. The function of phase 5 is to restore as fast as possible the input voltages V+, V- of amplifier 12 to their initial values. In phase 5, switches S1 and S4 are off. Amplifier 12 thus returns to its initial configuration for regulating the quiescent voltage. Additionally, switch S6 that shorts resistor R2 is turned on. Thus, an equilibrium state of amplifier 12 is very rapidly reached due to the fast charging of the capacitor C1. As is shown in FIG. 3, the voltage signals V+ and V- rapidly recover their initial values.
Phase 5 must be sufficiently long so that voltages V+ and V- recover their initial values before the end of this phase. Phase 5 ends at a time t7 determined by the fact that the voltage VC2 across capacitor C2, that is still charged by source I3, reaches an adequately selected threshold value V2. Switch S6 is turned off and amplifier 12 is restored to its initial configuration operable for the slow regulation of the quiescent voltage. The read mode could be restored at time t7. In fact, it is restored at a subsequent time t8 when switch S7 is turned on again in order to enable the output of comparator 14.
A problem with the above described operation is that, due to the abrupt switching off of switch S6, a biasing current i of amplifier 12 has sufficient time for charging capacitor C1 before the regulation of the quiescent voltage begins. As represented by curve VH, voltage V- increases slightly, then starts decreasing slowly due to the quiescent voltage regulation.
As represented by curve H, the output of amplifier 12 is the sum of the voltage signals (V+)-(V-). The output of the amplifier is not accounted for between time periods t2 and t8 due to the fact that switch S7 remains off between these time intervals. Time t8 is determined by the fact that voltage VC2 across capacitor C2 reaches a threshold value V3 slightly lower than the end value reached by voltage VC2 (approximately equal to VCC).
At a time t9, the next rising edge of signal C occurs. As represented by curve H, the output voltage of amplifier 12 is, at time t9, still below its quiescent value, due to the parasitic charging of capacitor C2 by current i. If at the time t9 the output voltage of amplifier 12 is still too far from its quiescent value, the pulse corresponding to the rising edge of signal C may not be detected by amplifier 14. In practice, less than 40% of the period of signal C, i.e., approximately 15 milliseconds, is available to stabilize the output of amplifier 12 to an adequate value (allowing for the detection of the next rising edge of signal C) after a write operation.
However, the present invention is directed to lessening the time required to stabilize the read head after a write mode, which becomes difficult with the conventional above-described circuit.